Various techniques are already known for piece molding of various articles from foamable plastic or resin materials in which a batch of the foamable thermoplastic material is introduced into a mold for instance by injection of the softened thermoplastic material. The mold is cooled and after hardening of the resin the mold is opened and the piece removed. In this way articles are made having a substantially impervious or unexpanded sheath or shell, usually in the form of a relatively thin surface layer.
Certain techniques have also been known using an extruder for feed of material and an independently operating puller mechanism for take-off, and providing for the continuous production of profiles. As with known piece-by-piece operations these operations produce profiles the surface layer of which is of relatively small thickness so that it does not contribute substantial structural strength to the profile.
In addition to inability to produce a skin or shell of substantial thickness and substantial structural strength, prior known techniques for continuously producing such profiles have also been subject to certain other disadvantages, for example, design of the forming die so that high frictional effects when passing material therethrough cause breakage of the profile, especially because of the independent and thus uncoordinated operation of the pulling mechanism and the extruder.
In addition the prior techniques have not been capable of selective control so that the extent of expansion of the core and the thickness of the unexpanded structural shell may be determined at will.
In addition certain prior techniques for continuously producing profiles having a porous core and an impervious surface have been sensitive to fluctuation of certain operating conditions, so that difficulty has been experienced in maintaining the continued operation of the system without encountering undesirable variations of the characteristics of the product produced. In some cases the sensitivity of the system has resulted in breakage of the piece being made with consequent shut down of the operation.
Efforts have also been made (see Kuhlmann French Pat. No. 1,498,620) to produce profiles having a porous core and a relatively thick and strong unexpanded skin by forcing the extrudate through a forming die by means of an extruder, but this technique is disadvantageous because the rate of production is very low.
It is a primary object of the present invention to provide a technique for producing foam profiles which are truly structural in character, the surface layer of the profile being of such thickness and density as to provide a high degree of structural strength. The invention moreover provides for the manufacture of such profiles at a high production rate.
It is a further object of the invention to provide for a technique for making such truly structural foam profiles under conditions which may be controlled to regulate important characteristics of the product such as extent of foaming of the core, thickness and density of the shell, and overall weight or composite density of the product measured, for example, in pounds per cubic foot (pcf).
It is a further object to provide a technique for producing structural foam profiles having good dimensional stability.
In considering another object of the invention it is first to be noted that, as is known, the operation of extruders may fluctuate somewhat as to temperature or pressure of the material supplied thereby. The invention provides a control system adapted to maintain uniformity of characteristics of the profile being made, notwithstanding fluctuations in the operation of the extruder.
Although the regulation or control system may be manually adjustable, the invention contemplates the provision of automatically operating means for regulating the production, for instance in accordance with the cross sectional size of the mass of expanding resin material in the unconfined zone lying between the discharge orifice of the extruder and the entrance orifice of the sizing die.
Another object of the invention is to provide a technique for the production of some profiles which is not only accurately controllable, but which is reliable and capable of continued uninterrupted operation for long periods of time.
It is still further contemplated to produce structural foam profiles having many of a wide variety of cross sectional shapes, including not only simple shapes such as cylindrical rods and square or rectangular bars or boards, but also more complex shapes, for instance board-like pieces having angles or grooves therein. It is further contemplated according to the invention to provide for the production of profiles having a hollow interior, for instance pipes in which the profile is provided with a dense structural surface layer not only on the outside surface of the pipe but also on the inside surface, the core intervening between the outside and inside surface layers being porous or expanded.
Because the technique of the present invention is capable of controlled operation in various respects referred to above, it is also possible according to the present invention to produce structural foam profiles having characteristics so closely resembling those of wood that the profiles may readily be employed in substitution for wood pieces as in the case of boards, moldings, and the like. Such profiles according to the invention may readily be cut by hand or a power saw in the manner of wood pieces of similar size and shape. They may be drilled by means of hand or power drills in the manner of wood pieces. They may also be fastened to other pieces of the same kind or to wood by nailing or screwing, nailing operations being capable of use without any appreciable tendency to split or otherwise damage the piece.
According to the technique of the invention, it is also possible to produce such structural profiles which not only have the characteristics of wood but which even have graining effects at the surfaces simulating certain woods, which is of advantage in some situations where the pieces are to be used in combination with wood pieces. The grain effect can include surface pigmentation to resemble the multi-colored grain of various woods.
Still another characteristic of profiles made in accordance with the technique of the present invention is the fact that the surface texture of the profiles is such as to readily accept painting and other decorative surfacing materials in much the same manner and by the same techniques as are usable with wood.
Finally, the profiles made according to the present invention not only have various of the characteristics referred to above, but these profiles may readily be produced at a cost which is at least comparable with the cost of many woods and which in many cases is even less expensive than comparable woods. The scarcity of suitable dimensionally stable wood has caused increases in price, making the present profiles comparable in cost. In the case of wood profiles of more complex cross section, the cost of shaping and finishing the wood makes the present invention even more attractive from a cost standpoint.
How the foregoing objects and advantages are attained, together with others which will occur to those skilled in the art will appear more fully from the following description referring to the accompanying drawings.